LTE (Long Term Evolution), sometimes referred to as 3.9G, 4G or Evolved Packet System (EPS) is a new and developing Standard in the mobile (cellular) telecommunications network technology. LTE operates entirely in the packet switched (PS) domain, in contrast to 2.5G (GPRS/EDGE) and 3G (UMTs) networks which operate in the circuit switched (CS) and packed switched domains.
Short Message Service (SMS) is the text and (small) data communication service of mobile communication systems, using standardised communications protocols that allow the exchange of short text/data messages between fixed line or mobile devices. SMS text/data messaging is the most widely used data application in the world, currently with 2.4 billion active users.
SMS as used on modern terminals is derived from radio telegraphy in radio memo pagers using standardised protocols, and was defined as part of GSM Standards as a means of sending messages of up to 160 characters or 140 bytes of data, to and from mobile handsets. Support for the service has expanded to include other mobile technologies such as UMTS, ANSI, CDMA and Digital AMPS, as well as satellite and fixed landline networks. Most SMS messages are mobile-to-mobile text messages, though the standard supports other types of broadcast messaging.
As well as conveying text messages between users, SMS is also used for sending notifications to users from the mobile network, such as notifications of roaming costs.
Additionally SMS is used to perform Over The Air (OTA) updates of mobile terminals or their SIM cards—such as to update the preferred networks a mobile terminal will use when roaming. As the size of SMS messages is restricted, updating a list of preferred roaming networks may require a multiplicity of SMS messages (e.g. up to 20 SMS messages). Such OTA messages are typically received and processed by a mobile terminal without the user being aware of any activity.
In 2G, 2.5G and 3G networks, SMS messages can be transmitted in the CS domain or the PS domain. LTE was intended not to operate in the CS domain, so the handling of SMS messages must be processed in the PS domain, or, by another (non-LTE) radio technology. However, since LTE Mobility Management Entities (which will be briefly described below) do not support the legacy Core Network protocols to connect to the SMS system, this presents new challenges
The third generation partnership project (3GPP) has recently defined a new concept known as IMS (IP-based Multimedia Subsystem). The IMS is a set of core network servers sitting behind the GGSN (which will be briefly described below) in the packet switched domain. These servers are introduced in order to process signalling between end users. The aim of IMS is to allow users such as mobile telephone network operators to provide services to their subscribers as efficiently and effectively as possible. For example, the IMS architecture is likely to support the following communication types: voice, video, instant messaging, “presence” (a user's availability for contact), location-based services, email and web. Further communication types are likely to be added in the future.
This diverse collection of communication devices requires efficient session management due to the number of different applications and services that will be developed to support these communication types. The 3GPP has chosen Session Initiation Protocol (SIP) for managing these sessions.
The SIP protocol is a session-based protocol designed to establish IP based communication sessions between two or more end points or users. SIP is used for signalling, end-to-end, the initiation, modification and termination of packet switched sessions. Once a SIP session has been established, communication between these end points or users can be carried out using a variety of different protocols (for example those designed for streaming audio and video). These protocols are defined in the SIP session initiation messages.
With IMS, users are no longer restricted to a separate voice call or data session. Sessions can be established between mobile devices that allow a variety of communication types to be used and media to be exchanged. The sessions are dynamic in nature in that they can be adapted to meet the needs of the end users. For example, two users might start a session with an exchange of instant messages and then decide that they wish to change to a voice call, possibly with video. This is all possible within the IMS framework. If a user wishes to send a file to another user and the users already have a session established between each other (for example, a voice session) the session can be redefined to allow a data file exchange to take place. This session redefinition is transparent to the end user.
One proposal for handling SMS messages in LTE networks is to transmit them using IMS. However, IMS is a complex Standard, and will not be employed in all networks/handsets at least initially.
Another proposal for handling SMS messages in LTE networks is to use the “Circuit Switched Fall Back” mechanism defined in 3GPP TS 23.272, which is fully incorporated herein by reference. The “Circuit Switched Fall Back” mechanism was originally intended for voice call handling and envisaged the mobile temporarily moving from LTE access to 2G or 3G access. SMS was added as an after-thought and then ‘optimised’ such that a change of radio technology was not needed. As a result, the solution for supporting MSC based SMS in LTE is unnecessarily complicated for the purpose of SMS support alone, especially if the network does not support legacy 3GPP 3G or 2G access.
The current handling of SMS in TS 23.272 requires full MSC/VLR (Mobile services Switching Centre/Visitor Location Register) functionality that supports mobility management from a CS perspective. The MSC/VLR is an expensive piece of equipment. If “Idle mode” mobility between LTE and GPRS/EDGE Radio Access Network (GERAN)/UMTS Terrestrial Radio Access Network (UTRAN) (i.e. 2G/3G) networks is active, this will furthermore have impacts on the 2G/3G core networks in terms of signalling load or requiring deployment of the Gs interface.